Because mainstream breast cancer treatments were developed for adults (an estimated 180,000 women will be diagnosed this year), Hannah's chemotherapy must be tailored to her small size. "I feel like a kid inside, but sometimes I feel like an adult when I was at the hospital," Hannah said.

Hannah's mother Carrie Auslam told "Good Morning America" that the day her daughter was diagnosed has been the worst moment of this journey. Hannah had complained about an itch on her side and when Auslam looked, she felt a lump.

Doctors assured her it couldn't possibly be breast cancer, until the biopsy came back.

"She cried for about three minutes and she's had a couple of moments," Auslam said of her daughter. "But she stood up after we talked and she goes, 'Let's do it.'"

Dr. Marisa Weiss, president and founder of Breastcancer.org and author of the book "Taking Care of Your 'Girls': A Breast Health Guide for Girls, Teens, and In-Betweens (2008)," said there's a concern about pollutants, pesticides and hormones in certain foods and drinks and their effects on breast-cell growth.

"It is extremely rare to have breast cancer in a 10-year-old girl and we don't know exactly why this would happen," she said. "We think that estrogen ... has to play a role."

So, as Hannah began treatment, her family threw her a party to say good bye, for now, to her hair. Her family applauded and, in solidarity, her father, grandfather and little brother all got buzz cuts as well.

"It hurt," her mother said. "She shouldn't have to do it. I've told her a thousand times, 'I wish I could take it away from you. I really do.'"

Hannah's Cancer a Scary Diagnosis

Regardless of the kind, cancer is a heavy diagnosis to handle for a child Hannah's age, noted Lillie Shockney, administrative director of the Johns Hopkins Avon Foundation Breast Center in Baltimore.

"This is the youngest case I've ever heard of," Shockney said. "I find for youngsters at this age it's best to not be focusing on the kind of cancer it is, but that it is cancer and that surgery and other treatment are needed.

"It's hard enough for adults to get their heads around breast cancer, much less a child."

What's more, Shockney said, it's inadvisable for parents to believe breast cancer is a major risk for their young daughters.

"I don't want the outcome to be that mothers are panicked across the country wanting to have their daughters in elementary and middle and high school to get mammograms or even clinical breast exams," Shockney said. "This is a highly unusual situation."

Shockney was not the only one concerned about how to present Hannah's diagnosis to the public. While the Auslam family has been open about Hannah's struggle, the media coverage of her experience has sparked debate among breast cancer experts about whether a rare case of cancer should be given widespread coverage.

The risk of calling attention to such a story, said Fran Visco, president of the National Breast Cancer Coalition, is that parents and their daughters are left with the impression that this form of cancer is far more common in young girls than it actually is -- a misconception that she said could lead to unnecessary fear and unneeded medical procedures.

"Breast cancer in this population is exceedingly rare, less than one in a million," Visco said. "We don't want these girls doing self-exams and getting mammograms. We don't want to spread that message; it will only result in harm and no benefit. It's important to understand: this could be harmful to these young women."

Cancer in Young Girls 'Not a Public Health Issue'

Dr. Daniel Hayes, clinical director of the Breast Oncology Program at the University of Michigan Comprehensive Cancer Center in Ann Arbor, agreed that there was no need for young girls to begin unnecessary cancer screening.

"This is not a major public health issue, and there is no 'take-home' message," he said. "What we don't want is a bunch of young girls having unnecessary biopsies or, worse, mammograms."

But Breastcancer.org's Weiss, who also heads up the Prevention Initiative for girls, a national outreach program designed to educate girls ages 8 to 18 on breast health and breast-cancer prevention, said she firmly supports the idea of educating girls about breast cancer and how to prevent it, even at a young age range.

At the same time, she noted, "the fact is that breast cancer in girls is extraordinarily rare, though it does happen."

Still, Weiss said that educating girls when they are young lays the foundation for a lifetime of breast health.

"What we do is explain to them what to expect in terms of breast development, what are healthy changes, and what are unhealthy changes."

Weiss added that establishing positive breast health habits when girls are young -- habits such as healthy eating, exercise and smoking avoidance -- has the potential to lower future breast cancer rates.

"There is an opportunity, while they are that age, to build the healthiest foundation possible ... this is just about the time when they begin to take up those unhealthy behaviors."

Regardless of how rare Hannah's cancer is, some doctors said the case illustrates the need for parents to communicate their concerns to doctors, and for doctors to take into account any potential health threats, however unusual.

"What bears emphasizing is that: a) this is incredibly rare, and teenagers need not worry about this happening to them; and b) physicians need to be aware that, while rare, this can happen, so that new lumps should be taken seriously," said Dr. George Sledge, professor of Medicine and Pathology at the Indiana University School of Medicine in Indianapolis and editor-in-chief of the journal Clinical Breast Cancer.

Michelle Schlief and the ABC News Medical Unit contributed to this report.